Coal gasification

ABSTRACT

A method of producing syngas wherein a carbonaceous feedstock is exposed to a plasma arc generated by a DC supply in a dry-steam environment.

BACKGROUND OF THE INVENTION

This invention relates to the gasification of coal.

Gasification is a process which is carried out by contacting a carbonaceous fuel material and steam under suitable conditions of temperature and pressure in order to form syngas, which is a mixture of carbon monoxide, hydrogen and carbon dioxide. Various types of gasification processes which have been proposed include, at least, a counter-current fixed bed gasification process, a co-current fixed bed gasification process, a fluidized bed gasification process, an entrained flow gasification process, an in-situ underground gasification process, and a plasma gasification process. These processes are dependent, at least, on the manner in which the steam and the carbonaceous material are fed to a gasifier and on the characteristics of a burden in the gasifier.

In a plasma gasifier, constituted with a number of plasma torches, a high voltage current is fed to each plasma torch thereby creating a high temperature plasma gas stream. A plasma torch can be operated with a number of gases including nitrogen, argon, steam and carbon monoxide. The size of a plasma torch is restricted by its power output and its operating lifetime.

When a plasma torch reactor is used for the gasification of coal, process energy is supplied by the plasma torches only and no coal combustion is carried out. Thus the calorific value of the coal is irrelevant. However the volume and quality of the syngas as well as the economic viability of the process depend on the carbon content of the coal and on the process efficiency. The latter parameter is determined by the rate of gasification.

In a conventional gasification process, a portion of the coal is combusted with oxygen to form carbon dioxide and small amounts of carbon monoxide, in order to produce the required process energy. In a plasma process the oxygen is replaced by a plasma gas, such as nitrogen or argon, which is produced in a gas separation plant which is similar to an oxygen plant. Raw syngas diluted with the plasma gas is generated. To the applicant's knowledge, commercial plasma torch reactors have been implemented only for the gasification of municipal solid wastes.

An object of the present invention is to implement coal gasification using plasma arc techniques derived from a metallurgical direct-current (DC) plasma arc furnace.

SUMMARY OF THE INVENTION

The invention provides in the first instance a method of producing syngas wherein a carbonaceous feedstock is exposed to a plasma arc generated by a DC supply in a dry-steam environment.

Preferably the carbonaceous feedstock is coal and, for example, is a low grade or discard coal.

The plasma arc may be generated by the use of equipment similar to that employed in a DC open arc furnace. The equipment may include multiple graphite electrodes of solid or hollow design.

The plasma arc may be produced in a reactor and the carbonaceous feedstock may be introduced via a plasma arc plume into a lower region of the reactor. In a variation the feedstock is introduced into an upper region of the reactor in an entrained flow section of the reactor. The carbonaceous feedstock may be fed together with a dry steam feed into the reactor or it may be fed separately, from a dry steam feed, into the reactor. If hollow electrodes are used the feedstock may be introduced into the reactor through these electrodes.

The plasma arc may be an open arc, or the electrodes may be immersed in slag in a bottom region of the reactor i.e. immersed-arc heating may be implemented. Open-arc heating and immersed-arc plasma heating may be combined.

The invention further extends to an apparatus for producing syngas from a carbonaceous feedstock material which apparatus comprises a DC arc gasifier, which processes the feedstock material and which includes a lower vessel within which a plasma arc is established and an upper vessel which is operated as an entrained flow gasifier.

The plasma arc may be established as an open arc or an immersed arc. In the latter instance the immersed arc may be produced in a molten slag layer in a lower region of the lower vessel.

The feedstock material, i.e. the carbonaceous material, may be supplied to the lower vessel, or to the entrained flow gasifier which, in turn, feeds into the lower vessel.

The apparatus may include one or more electrodes which may be hollow and the feedstock may be introduced into the lower vessel through the electrodes.

The lower vessel may include a slag containment structure with a slag tapping facility and at least one electrode which establishes a DC plasma arc in the lower vessel.

The apparatus may be operated at atmospheric pressure, or at a pressure which is higher than atmospheric pressure.

BRIEF DESCRIPTION OF THE DRAWING

The invention is further described by way of example with reference to the accompanying drawing which illustrates from one side and in cross section a DC arc plasma gasifier according to one form of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

In implementing the invention use is made of a structure which is similar to a multiple electrode DC open arc furnace. With the adoption of this concept a larger gasification capacity is possible. As indicated, plasma-based gasification does not require oxygen and a gas separation unit is not required. It is anticipated that the exclusion of these features will lower the capital cost of the plant.

During gasification the process temperature is a principal parameter in acting on the kinetics and on carbon conversion. The process temperature depends on the nature of the gasification structure and on design factors with an object being to achieve a maximum and efficient transfer of energy from the plasma arc by means of radiation and convection.

The accompanying drawing illustrates a DC arc gasifier 10 according to the invention which includes a lower vessel 12 and an upper vessel 14 which is operated as an entrained flow column 16.

The lower vessel 12 includes a slag containment structure 20 with a cover 22. Multiple electrodes 24, 26 etc. pass through respective ports in the cover. Seals 28 are used to seal respective interfaces between the cover 22 and each electrode 24, 26, etc.

The structure 20 has slag tap holes 30 at strategic locations.

A slurry 36 of pulverised coal 38 and dry-steam 40 is introduced into the entrained flow column 16 at an appropriate position on a side of the column. As is described hereinafter, the slurry 36 could also be introduced via an arc plume (not shown) below the cover 22, into the containment structure 20. The feeding position should be appropriately implemented to optimise efficiency of gasification. Both feeding arrangements can be adopted.

At a lower end 44 the entrained flow column 16 discharges process residues (not shown) through the cover 22 into the containment structure 20.

The upper vessel 14 is operated as an entrained flow gasifier. The pulverised coal 38 and the dry steam 40 are contacted inside the vessel 14 in co-current flow. Gasification reactions take place within a cloud of fine particles. Low grade coal and discard coal are suitable for this type of gasification because of the high operating temperature, in the vessel 14, which is created by the plasma arc. The high temperature, the prevailing pressure inside the vessel 14, the residence time, the steam-to-coal ratio, and the steam characteristics, are parameters which are selected to achieve a higher throughput of feedstock. Due to the aforementioned conditions tar and methane (volatile) are not present in the product gas.

Additional heating may be performed in the entrained flow gasifier 16 by partially combusting a gas stream which is generated in the lower vessel 12 with oxygen. This helps to reduce electricity consumption in the entrained flow gasifier 16.

The coal/steam slurry 36 is introduced into an upper region inside the structure 20. It is possible, though, to introduce the slurry 36 into a lower region inside the structure 20, or into the upper region and into the lower region. Nozzles, not shown, are optionally used at the lower, discharge end 44 of the entrained flow column 16 to direct the coal/steam slurry 36 into the interior of the structure 20. Alternatively or additionally, as is notionally indicated by dotted lines 50, the coal/steam slurry 36 is fed to the interior of the structure 20 through the electrodes 24, 26 etc. For this purpose each electrode is formed with an elongate tubular bore 52 which is lined with a layer of protective material, e.g. a ceramic sleeve, which is inert to steam and which is resistant to thermal shock. In use of the apparatus the slurry 36 is thus fed, in an open arc plume, into the interior of the structure 20.

The electrodes 24,26 are shown in an open arc configuration i.e. with lower ends displaced from a slag melt 56 which accumulates in a lower region inside the structure 20. A high temperature reducing condition is established by the open arcs in the vessel 12 above the slag melt 56. It is possible, though, to immerse lower ends of the electrodes 24, 26 into the molten slag 56 so that the coal gasifier operates as a slag-based, immersed-arc, reactor. A hybrid concept is possible, i.e. a gasifier in which use is made of immersed-arc, and open-arc plasma, heating.

It is important to be able to control various process parameters during the operation of the gasifier 10 in order to achieve effective operation. These parameters include the magnitudes of the voltages and currents supplied to the electrodes, and the power consumption and the stability of the arcs inside the vessel 12 in the presence of steam or syngas, or in the presence of steam and syngas in various proportions.

During the operation of the gasifier 10 the temperature inside the upper vessel 14 should be maintained above 600° C. to achieve efficient coal gasification. Higher temperatures are preferable since they increase the rate of gasification. The energy within the upper vessel 14 comprises the enthalpy of the gas product flowing through it.

The operation of the metallurgically-based gasifier 10, in the lower vessel 12, is similar to that of an immersed arc smelter. Most of the arc energy is transferred to the molten slag bath 56 by convection. The pulverised coal 38 and dry steam 40 are injected into the arc plume or plumes above the bath 56. It is desirable to make use of dry-steam to limit the effects of electrode corrosion and to help to stabilise the arc. In a molten bath coal gasification process, heat transfer to a reaction zone and the extent of the contact between the coal and the steam, might limit the kinetics and reactor throughput. In this instance the upper vessel 14, which is operated as an entrained flow gasifier 16, provides additional throughput. Thus the lower vessel 12 is used to produce syngas which is combusted to provide energy for the gasification of coal in the entrained flow gasifier 16.

The gasification process takes place in a steam plasma. The behaviour of water molecules and, particularly of hydrogen molecules, is relevant as such behaviour can dominate the plasma composition. Steam plasma requires more energy to dissociate than either air, or a carbon monoxide plasma. The net effect is that a steam plasma has a higher electrical resistivity than air, or a carbon monoxide plasma. The introduction of steam into a reaction zone thus causes a meaningful change in resistivity. This must be done, though, in a manner which does not adversely affect arc stability. A longer arc is established, due to the higher arc resistivity brought about by the dry-steam environment, and the electrodes thus require a higher voltage.

The process product i.e. the syngas is drawn from an upper region of the lower vessel 12 via a port 60.

The present invention has several advantages over existing incinerator or plasma torch gasification reactor designs. A first benefit is that a separate plant to produce pure oxygen as a feedstock for the reactor is not required, thereby saving capital and operating costs. A second benefit lies in the fact that the plasma arc operates at an extremely high temperature, at least of the order of 10000° C., resulting in syngas of an improved quality. Thirdly, a metallurgical DC furnace uses proven and efficient technology at a much higher operating power, up to 80 MW, than can be achieved with a plasma torch reactor. This means that a DC gasifier based on techniques associated with a metallurgical DC furnace, are expected to produce economy-of-scale benefits. 

1-11. (canceled)
 12. A method of producing syngas wherein a carbonaceous feedstock is exposed to a plasma arc generated by a DC supply in a dry-steam environment in an apparatus which includes a lower vessel and an upper vessel, wherein the carbonaceous feedstock is introduced directly into the lower vessel via a plasma arc plume or via the upper vessel which is operated as an entrained flow gasifier.
 13. A method according to claim 12 wherein the carbonaceous feedstock is fed together with a dry steam feed into the apparatus or it is fed separately, from a dry steam feed, into the apparatus.
 14. A method according to claim 12 wherein the plasma arc is at least one of the following: an open arc within the apparatus, and an immersed arc produced by immersion of an electrode into slag in a lower region of the apparatus.
 15. An apparatus for producing syngas from a carbonaceous feedstock material, which apparatus comprises a DC arc gasifier which processes the feedstock material and which includes a lower vessel within which a plasma arc is established and an upper vessel which is operated as an entrained flow gasifier wherein the feedstock is fed to the lower vessel through a) at least one hollow electrode or b) via the entrained flow gasifier.
 16. An apparatus according to claim 15 wherein the plasma arc is established as at least one of the following: as an open arc, and as an immersed arc in a molten slag layer in a lower region of the lower vessel.
 17. An apparatus according to claim 15 wherein the lower vessel includes a slag containment structure with a slag tapping facility and at least one electrode which establishes a DC plasma arc in the lower vessel.
 18. An apparatus according to claim 15 wherein pulverized coal, which is the carbonaceous feedstock material, is applied to the DC arc gasifier in a slurry formed from the pulverized coal and dry steam. 